Within a single generation time a growing yeast cell imports ∼14 million ribosomal proteins (r-proteins) into the nucleus for ribosome production. After import, it is unclear how these intrinsically unstable and aggregation-prone proteins are targeted to the ribosome assembly site in the nucleolus. Here, we report the discovery of a conserved nuclear carrier Tsr2 that coordinates transfer of the r-protein eS26 to the earliest assembling pre-ribosome, the 90S. In vitro studies revealed that Tsr2 efficiently dissociates importin:eS26 complexes via an atypical RanGTP-independent mechanism that terminates the import process. Subsequently, Tsr2 binds the released eS26, shields it from proteolysis, and ensures its safe delivery to the 90S pre-ribosome. We anticipate similar carriers—termed here escortins—to securely connect the nuclear import machinery with pathways that deposit r-proteins onto developing pre-ribosomal particles.DOI: http://dx.doi.org/10.7554/eLife.03473.001

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Activated Mpk1 MAPK regulates Swi6 nucleocytoplasmic shuttling in a bi-phasic manner. First, it recruits Swi6 to the nucleus non-catalytically by forming a nuclear Mpk1/Swi4 complex to initiate cell wall stress transcription. Then, it phosphorylates Swi6 to prevent further nuclear entry, resulting in its relocalization to the cytoplasm.

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the endoplasmic reticulum (ER) membrane where neutral lipids are synthesized. While the molecular mechanisms of this process are unknown, it appears that LD biogenesis is following a threestage process: neutral lipid synthesis, lens formation (via intra-membrane lipid accumulation), and finally LD formation. [3] At relatively low concentrations, neutral lipids accumulate between the two leaflets of the ER bilayer to eventually form a lipid reservoir with a lens shape. [3,4] Numerical modeling suggests that such lenses could exist in the ER membrane with a size of tens of nanometers. [3,5,6] Upon the continuous production of neutral lipids, they accumulate into the reservoir which leads to a lipid lens growth. Above a certain size, this lipid lens becomes unstable and can lead to a spontaneous budding of the oily reservoir. [3,7] Ultimately, the droplet pinchoff from the membrane leads to its release into the cytosol. [8][9][10] LDs dynamically adapt to metabolic changes in the cell and balance uptake of free fatty acids by their esterification and storage as triglycerides, and consumption of triglycerides by the release of fatty acids under catabolic conditions. These essential metabolic functions of LDs are executed by proteins on the LD surface, many of which dynamically partition from the ER bilayer to the LD monolayer membrane. [11,12] Aberrant LD functions are implicated in numerous metabolic diseases such as obesity, diabetes,The ORCID identification number(s) for the author(s) of this article can be found under